Author

Date of Award

Degree Name

Department

Chemical and Paper Engineering

First Advisor

Dr. Brian Young

Second Advisor

Dr. Massood Zandi Atashbar

Third Advisor

Dr. Margaret Joyce

Fourth Advisor

Dr. James Springstead

Abstract

This study supports the development, characterization and optimization of biosensor material formulations for immunodiagnostic applications based on experimental findings and hypotheses by Wang and Wu [1, 2], and using a test-plate apparatus and thin-film design developed by Young [3].

Certain biosensors working on the basis of conductance/impedance changes have demonstrated their potential to detect various bacteria, enzymes, and biomolecules due to their enhanced electrical properties with incorporated single-walled carbon nanotubes (SWNT). From the work conducted in our laboratory, we are attempting to determine and develop a matrix formulation containing highly dispersed SWNT, antibody and other components that elicit a significantly larger change in matrix conductivity with antigen binding than would occur from only electrochemical impedance effects.

One of the most challenging aspects of using SWNT in biosensors is uniformly dispersing them within the sensing component. The dispersion of the SWNTs in an aqueous solution is particularly challenging. In aqueous solutions, cylindrical SWNTs tend to form aggregates/bundles, appearing as a micro-meshwork or a network of ropes. The resulting semisolid solutions therefore do not often show the anticipated electrical properties after solution deposition and drying.

Studies included in this dissertation focus on obtaining uniform SWNT dispersions (using a well characterized commercially available conductive SWNT) with either poly(sodium 4- styrenesulfonate) (PSS) or carboxymethylcellulose (CMC), two dispersing agents, and other components in aqueous solutions using various sonication protocols. The high/low values used for the statistical design ranges chosen were based on preliminary work that showed roughly where SWNT dispersion was visually incomplete or dried coated matrix film resistances exceeded the limits of the test equipment used. Once characterized, SWNT dispersions were selected based on resistance level, processing attributes and film characteristics and used in a second phase of the research by combining with antibody (for future use as a capture molecule in biosensing) and glycerin (added as a wetting and perhaps antibody stabilizing agent). These SWNT dispersions together with varying levels of antibody and glycerin using a statistical design protocol were deposited within test sensor channels, dried into a semi-solid/fluid film, and resistance-tested as before.

Formulated solutions were characterized using dynamic light scattering. Dried, semisolid/ fluid composite “matrix” films deposited on test plate templates were characterized using 3D optical microscopy. Visualizing the structural aspects of the matrix film allows for a better understanding of physical factors that may affect the resistivity of the matrices, perhaps allowing for better predictability of resistance with parameter changes. Film characterization with scanning electron microscopy was also attempted, but with very limited success perhaps due to the presence of glycerin within the sample.

Printing technologies consisting of screen printing and material plotting were used to create various elements of the test plate used in these studies. From the results obtained in this study, it was found that run 15 matrix solution (SWNT:CMC=2, sonication amplitude=40%, sonication duration=24 hr) resulted in dried matrix films with the lowest resistance (1.23×10-02Ω.cm) and possessed an average particle size of ~115 nm. In contrast, the largest particle size (where 81% of the particles have an average particle size of ~5000 nm), for the set of formulation experiments in which CMC was used, was observed in Run 3 (SWNT:CMC=0.5, Sonication amplitude=20%, and sonication duration= 2 hr) in which the highest dried film resistivity was obtained. From this it was concluded that a lower SWNT:CMC ratio (lower conductive:insulating components) is contributing to larger SWNT particle size that together are contributing to higher film resistivity. The sonication factor was also found to play an important role in the dispersion and conductivity of SWNTs. The analysis of variance of the particle sizes obtained in the PSS-dispersed matrix solutions shows that the sonication amplitude and its interaction with the concentration of PSS are the only significant parameters among all that parameters were studied (α=0.05).

Profilometry of Run 3 (CMC-dispersed coating) at 100% (no dilution) showed an average roughness of 0.274 μm while profilometry of Run 15 at 100% (no dilution) showed an average roughness of 0.067 μm. The same trend was noted for matrix coatings at higher dilutions (70% and 40%). Therefore, increased particle size of the matrix formulations had a strong correlation with increased roughness, and increased resistance of the CMC-dispersed coatings. The thickness of the dried matrix films was found to decrease with the dilution of the matrix solution as would be expected due to reduced solids content. Calculated thickness values based on mass balance results for each prepared solution were compared with thickness measurements by the 3D optical microscope. The calculated values were found to be very close to the experimentally measured thickness measurements.